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Say your backpacking for days and then you get a chance to put the backpack down and carry a day pack for a day. That's slackpacking.
I'd done the AT from Georgia to NJ/NY backpacking, stayed in a hostel in NY and the host asked if wanted to slack the next day, I too asked what slacking was. I've slacked as much of the trail as I can after learning what it is, which is to say I never met a backpack I truly loved!
Now in the Smokies there's not much slacking

The person "slacking" you would drop your gear off at a pre-determined meeting point or pick you up and take you to the hostel/camp for the night. Slacking lets you crank out miles like a day hiker but camp like a backpacker.

Usually a hostel along the AT will drop you off up the trail and you hike back to your stuff at the hostel, next day they'll drive you to where you got off the trail. Some you can call when you 10-20 miles out and they'll pick you up or your backpack.
Some hikers have slacked the entire AT save the 100 Miler Wilderness, one section in Vermont, and the Smokies where road crossings are few.
The young woman with the AT speed record essentially slacked (if you call serious trail running slacking!) from road to road where she had a swag.
Swagging=hiking/cycling to a van with food/shelter.